A growth method to obtain large size periodically-poled KTiOPO4 crystals
Résumé
In the last decades, attention has been focused in obtaining periodically poled ferroelectric crystals to get efficient nonlinear frequency conversion processes using Quasi-Phase-Matching (QPM). The electric field poling is the classic method to obtain the periodic inversion of the ferroelectric domains. The grating periodicity is very well controlled, but it is difficult to obtain thick samples: tipically 500 µm for periodically-poled-LiNbO3 (PPLN), 5 mm for periodically-poled-MgO:LN and between 1and 3 mm for periodically-poled-KTiOPO4 (PPKTP) . The limited aperture size of the samples forbids the use of beams with high energy according to the damage threshold of the crystals Several groups previously tried to obtain large size PPLN or PPKTP crystals by crystal growth processes from single domain seeds. Czochralski and Top Seeded Solution Growth techniques had been then used, proceeding by a periodic change of the growth conditions , . But in both cases, the regularity of the obtained grating period cannot be controlled, so that an efficient QPM frequency conversion cannot be achieved. With the aim of increasing the size of periodically-poled crystals with a controlled and regular grating period, we propose here a new protocol based on an epitaxial growth process using seeds made of thin periodically-poled plates previously obtained by electric field poling. By using a halide flux with the proper composition, we recently succeeded in the growth of a multi-domain PPKTP crystal with a grating period of 13 µm over a thickness of 1 mm from a PPKTP seed exhibiting {001} faces. It was necessary to perform the growth under the Curie temperature of the crystal in order to avoid the erasing of the domains grating, and below the roughening temperature of the {001} faces onto which the growth is performed. It is the first demonstration of that kind of crystal growth to the best of our knowledge.